This invention concerns improvements in and relating to fluorinated ion exchange polymers, and particularly to such polymers used in the form of films and membranes used in chloralkali electrolysis cells.
Fluorinated ion exchange polymer in such membranes can be derived from a fluorinated precursor polymer which contains pendant side chains in sulfonyl fluoride form. The sulfonyl fluoride functional groups have been converted to ionic form in various ways, for example, to sulfonate salts by hydrolysis with an alkaline material, to the sulfonic acid by acidification of the salts, and to the sulfonamide by treatment with ammonia. Examples of such teachings in the art can be found in U.S. Pat. Nos. 3,282,875, 3,784,399, and 3,849,243.
Although such polymers and membranes have many desirable properties which make them attractive for use in the harsh chemical environment of a choralkali cell, such as good long-term chemical stability, their current efficiencies are not as high as is desired, especially when the caustic is produced at high concentration. As transport of hydroxyl ion in a choralkali cell from the catholyte through the membrane to the anolyte increases, current efficiency drops. Larger amounts of oxygen impurity in the chlorine are thereby produced, and there is a greater buildup of chlorate and hypochlorite contaminants in the brine, which contaminants must be removed and discarded to maintain acceptable cell operation. Current efficiencies of at least 90% are highly desirable.
Accordingly, there is a need for polymers and membranes which will permit cell operation at high current efficiencies, and especially for those which will permit operation at high efficiencies over long periods of time. Additionally, it was desired to find a method for modifying the known polymers and membranes which have pendant side chains in sulfonyl fluoride form in such a way to obtain polymers and membranes which will have the high current efficiencies desired.